1. Field of the Invention
The present invention relates to a process for producing mesophase pitch-based carbon fibers. More specifically, it relates to a novel stabilizing method which is intended to improve the strength and the efficiency of the stabilization of the mesophase pitch-based carbon fibers.
2. Description of the Related Art
Currently, mainly PAN (polyacrylonitrile)-based carbon fibers are used for composite materials, but the PAN-based carbon fibers are expensive, because the starting PAN fibers are expensive, and have a low carbon yield, and therefore, have been used mostly in special fields related to sports and leisure, or aeronautics and space.
Conversely, pitch-based type carbon fibers obtained from carbonaceous pitch as the starting material have the specific features of a low cost production, because the starting material is inexpensive, and has a high carbon yield. Particularly, mesophase pitch-based carbon fibers obtained from a mesophase pitch containing 40% or more of mesophase, preferably 60% or more of mesophase as the starting material are now attracting attention as inexpensive materials which may provide high performance carbon fibers. With regard to mesophase pitch-based carbon fibers, it is commonly known that carbon fibers having a high preferred orientation and a high graphitizability can be easily produced, and therefore, fibers having a high Young's modulus can be produced. For example, Japanese Unexamined Patent Publication (Kokai) No. 49-19127 discloses mesophase pitch-based carbon fibers having a structure possessing the three-dimensional order characteristics of polycrystalline graphite, a high graphitizability, and an excellent modulus, and processes for producing same. Nevertheless, although such carbon fibers having a high graphitizability have a high Young's modulus, they have a drawback in that the tensile strength and the elongation at break are low. Accordingly, it is considered difficult to improve the tensile strength of mesophase pitch-based carbon fibers, even though it has an excellent Young's modulus.
Recently, various attempts have been made to improve the tensile strength of the mesophase pitch-based carbon fibers by controlling the structure thereof. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-104927 discloses that mesophase pitch-based carbon fibers having a lower graphitizability can be produced by stirring at a location immediately above the capillary of the spinning nozzle in the spinning, thereby producing a finer structure in the cross-sectional direction while maintaining a high preferred orientation in the axial direction, and that the tensile strength of the carbon fibers can be improved while maintaining a high Young's modulus. Investigations into an improvement of mechanical properties by a structural control of the mesophase pitch-based carbon fibers, however, have been primarily concerned with the spinning, and no reports have been given regarding the stabilization, carbonization, and graphitization. Also, the structural control in the mesophase pitch-based carbon fibers of the prior art is intended to provide a macroscopic structural control or a microscopic structural control of the whole parts of fiber, and no reports have been made of attempts to improve the mechanical properties by a microscopic structural control of a specific site, for example, by changing the structure of the surface layer of the fiber or changing the structure of the center portion.
For PAN-based carbon fibers, an attempt has been made to improve the mechanical properties by subjecting the fiber to electrolytic oxidation after a carbonization treatment, and then applying a heat treatment thereto in an inert gas, to thereby control the microscopic structure of the ultra-thin outermost layer of the fibers (Japanese Unexamined Patent Publication (Kokai) No. 61-225330). Nevertheless, investigations by the present inventors found that, even if this method is applied to the mesophase pitch-based carbon fibers, the mechanical properties can not be improved, and in some cases, the tensile strength was lowered. This may be considered to be due to the great difference in the structures of the PAN-based carbon fibers and the mesophase pitch-based carbon fibers.
Regarding the stabilization of pitch fibers, this is usually carried out in an oxidative atmosphere at a temperature of 100.degree. C. to 400.degree. C. Particularly, it is most usual to use air or a gas mixture of oxygen and nitrogen as the oxidative atmosphere, although attempts have been made to employ another oxidative gas such as nitrogen oxide as the oxidative atmosphere. The main purpose for these attempts was not primarily to improve the mechanical properties of the carbon fibers, but to shorten the treatment time. This is because the stabilization process is time consuming. For example, it takes a relatively longer treatment time of about 60 to 400 minutes in an air atmosphere. Particularly, in the case of an isotropic pitch-based carbon fibers, a prolonged treatment is required because the treatment must commence from a lower temperature at the initial stage of the stabilization. Japanese Patent Publication (Kokoku) No. 48-42696 discloses an example in which NO.sub.2 was used in the stabilization of the isotropic pitch-based carbon fibers, in which the treatment was conducted at a lower temperature of 30.degree. C. to 130.degree. C. and the treatment time was shorter than that for treatment using air. Also, as an example of an application of the mesophase pitch-based carbon fiber, Japanese Unexamined Patent Publication (Kokai) No. 60-259629 discloses a stabilizing treatment in an oxidative atmosphere such as air or oxygen containing 0.1% to 50% by volume of NO.sub.2, at a treatment temperature of 150.degree. C. to 380.degree. C. It is shown that a treatment time reduction and improvement of mechanical properties of the carbon fiber are obtained by this treatment. However, the present inventors found that the stabilization using of an oxidative atmosphere containing NO.sub.2 at a lower temperature of 100.degree. C. to 260.degree. C. is effective for a reduction of treatment time, but provides little improvement of the physical properties of the carbon fiber, and that the use of an oxidative atmosphere such as air or oxygen containing NO.sub.2 at a higher temperature of 260.degree. C. to 380.degree. C., results in a high oxidative consumption of the fibers, thereby causing a lower yield. Further, when an extreme oxidative consumption damages fibers, a problem arises also in that the mechanical properties may be lowered.
Therefore, in the stabilization of the masophase pitch-based carbon fibers, there are no reports on a method of controlling the structure of a carbon fiber, and the improvement of the stabilization rate is not satisfactory.